A phase I study of TRC105 (anti-CD105 [endoglin] antibody) in metastatic castration-resistant prostate cancer (mCRPC).
117 Background: Preclinical and clinical evidence demonstrates an important role for angiogenesis in mCRPC biology. CD105 (endoglin) is a transmembrane protein expressed on the surface of proliferating vascular endothelial cells. The expression of CD105 is required for the formation of new blood vessels. TRC105 is a human/murine chimeric IgG1 kappa monoclonal antibody that binds to human CD105 (endoglin). It inhibits angiogenesis and tumor growth through inhibition of endothelial cell proliferation, antibody-dependent cellular cytotoxicity, and induction of apoptosis. The primary objective is to evaluate safety and identify the maximum tolerable dose (MTD) of TRC105. Secondary objectives include the assessment of TRC105 pharmacokinetics, PSA response rate, evaluation of progression free survival (PFS), overall response rate (ORR) and overall survival (OS). Methods: Patients with an ECOG performance status (PS) ≤ 2, progressive mCRPC and either chemotherapy-naïve or post-docetaxel treatment were eligible. Five cohorts of patients, on escalating dose levels, receive TRC105 intravenously at doses of 1, 3, 10 or 15 mg/kg IV every 2 weeks (cohorts 1, 2, 3, and 5) or 10 mg/kg IV weekly (cohort 4) on a 4 week cycle. Response is assessed with imaging studies every 2 months for the first four months and then every 3 months thereafter. Results: Seventeen patients are enrolled in cohorts 1-5. Median age is 65 (range 48-87), median ECOG PS is 1 (range 0−2), median Gleason score is 8 (range 6−10), median on−study PSA is 147.5 (range 0.1-3373), and median number of prior (non-hormonal) therapies is 3 (range 0−6). Median time on study is 16 weeks (range 8-28 weeks). One patient experienced a dose limiting toxicity (grade 4 vasovagal episode) in cohort 5. PSA declines were seen in 6 patients ranging from 20% to 57% from baseline. Ten out of 12 patients with measurable soft tissue disease achieved stable disease for at least two cycles. Two patients remain on study (in cohort 5). Conclusions: TRC105 is tolerated up to 15 mg/kg every two weeks with early evidence of clinical activity in mCRPC. Accrual is ongoing to evaluate ORR, PFS, and OS in the phase II portion of this study.